The present invention relates to a highly heat-resistant positive-working photoresist composition or, more particularly, to a positive-working photoresist composition having high heat resistance as well as resistance against the conditions of dry ethching with stability of the pattern configuration of the resist layer, which is useful in the manufacture of various semiconductor devices and electronic components involving a step of photolithography.
As a trend in the electronic technology in recent years, semiconductor devices and electronic components are required to be manufactured in a more and more increased density of integration or compactness in size. Along with this general trend, the patterning works to form a fine resist pattern by the techniques of photolithography are also required to have increased fineness reaching the so-called submicron order which can be achieved only by a great improvement in the photoresist composition used in the process. As is known, photoresist compositions can be classified into negative and positive-working ones depending on the behavior of solubility changes induced by exposure to light. While it was the major current of the photolithography in the semiconductor industry of the early days that the photoresist composition was of the negative-working type, the current is now under shift toward replacement of the negative-working photoresist compositions with positive-working ones since the photoresist compositions of the latter type are more promising than the former in the possibility of giving a patterned resist layer of extremely high resolving power having a fineness of, for example, 1 to 2 .mu.m or even finer in the width of line patterns.
To briefly describe the photolithographic process by using a positive-working photoresist composition in the manufacture of semiconductor devices, the substrate such as a semiconductor silicon wafer is first coated with the photoresist composition in the form of a solution followed by drying to form a uniform layer of the photoresist composition which is then exposed pattern-wise to actinic rays such as ultraviolet light to give a latent image of the photoresist layer. The latent image of the photoresist layer is developed to give a patterned photoresist layer by use of a developer solution which dissolves away the photoresist composition only on the areas where the photoresist layer has been selectively exposed to actinic rays. The thus obtained pattern-wise photoresist layer on the substrate is usually subjected to a heat treatment to improve the heat resistance prior to the subsequent process of etching.
This process of etching is also on the trend of shift from the conventional wet-process etching to the dry-process etching such as plasma etching and reactive ion etching to meet the increasing requirements in recent years for the extreme fineness of patterning to reliably reproduce a very fine line pattern and for the automatization of the process. In this regard, high heat resistance is one of the essential characteristics of a positive-working photoresist composition used in the fine patterning works.
The conventional positive-working photoresist compositions of the major class in the prior art are usually formulated with a phenol novolac resin as a film-forming component which is admixed with or combined by an addition reaction with a photosensitizing component such as quinone diazide compounds. The photoresist compositions of this type are not quite satisfactory to meet the practical requirements in respect of the insufficient heat resistance due to the relatively low softening point of the component resin which limits the temperature of the heat treatment of the patterned resist layer prior to etching rarely to exceed 100.degree. to 110 .degree. C.
The heat resistance of a patterned layer of a positive-working photoresist composition after development can be improved alternatively by the irradiation with ultraviolet which is effective to form crosslinks between the molecules of the film-forming resin even when the resin is the above mentioned phenol novolac since the crosslinked resin has an increased softening point to withstand the heat treatment at a higher temperature than the resin before the photo-induced crosslink formation. Even by the combination of the ultraviolet irradiation and heat treatment, however, the patterned layer of the conventional photoresist compositions cannot be imparted with high heat resistance to withstand the conditions of dry etching process and fully comply with the recent requirements of fine patterning with little deformation of the patterned resist layer. Accordingly, it is eagerly desired to develop a novel positive-working photoresist composition having inherently improved resistance against heat and the conditions of dry etching.